In malignant animal tisses, both the activity and regulatory properties of HMG-CoA reductase, the rate limiting enzyme in cholesterolgenesis, are markedly altered such that: (1) total enzyme activity is increased; (2) more enzyme exists in an active, unphosphorylated form; and (3) feedback responsiveness to plasma cholesterol is absent or diminished. These abnormalities are early events in malignant transformation and may be critical to development of the malignant state. Reductase activity is also integral in controlling progression of normal cells through the cell cycle. We hypothesize that the rapid and uncontrolled growth rate of malignant cells is due to abnormal control of reductase activity and that these abnormalities may precede development of the malignant phenotype. This hypothesis will be addressed by: (1) comparing the regulatory properties of normal and malignant leukocytes; (2) comparing the time, magnitude and duration of reductase elevation during the cell cycle, using normal and malignant cultured human leukocytes synchronized in G-1 phase of the cell cycle by centrifugal elutriation; (3) examining changes in these properties in malignant leukocytes in response to tsreatment with reductase inhibitors, antimitotic agents or antileukemic drugs; and (4) examining changes in reductase regulatory properties and cell cycle involvement as cultured promyelocytic leukemia cells are induced to differentiate and as leukemic patients experience remission or relapse. Our recently developed methods for measuring human leukocyte microsomal reductase activity, reductase protein concentrations (by an enzyme-linked immunosorbent assay), and enzyme phosphorylation state will permit us to address thse questions in ways not previously possible. These studies should aid in our understanding of the relationship between cellular cholesterol metabolism and malignancy and may be important in determining whether abnormal regulation of reductase activity precedes development of malignancy. (B)